1. Field of the Invention
This invention relates to a bipolar output stage switching circuit and technique, suited for monolithic fabrication and intended particularly for use in automated test equipment to drive an input pin in an integrated circuit device under test (DUT). For digital use, the output of the circuit can be switched to a logic high signal, a logic low signal, or to a high impedance. The circuit is also adaptable to analog use and may be employed, for example, in the output circuit of an operational amplifier to provide a mode in which the amplifier ceases to act as a voltage source and instead provides a floating high impedance output.
2. Discussion of Prior Art
In testing digital integrated circuitry, a pin driver circuit is used to supply to the DUT the binary ones and zeroes needed to exercise the DUT. Thus, at a minimum, a pin driver circuit must be capable of providing logic high and low signals, corresponding to binary ones and zeroes, respectively, at its output. However, since some integrated circuits are intended to supply "read" data to the same pins to which "write" data is provided, on an alternating basis, it is necessary also to disconnect the pin driver circuit during read operations. Mechanically disconnecting the pin driver circuit while connecting a receiver circuit is a time consuming operation which slows down the testing process. It is advantageous, therefore, to be able to disable and disconnect the pin driver electrically from the DUT, without mechanically disconnecting it, allowing the pin driver output and the receiver circuit input to be in simultaneous electrical contact with the DUT. To this end, some pin drivers provide a mode in which the output of the circuit ceases to be a voltage source and instead becomes a high impedance which will not "load down" the output of the DUT. This capability facilitates reading from the DUT without disconnecting the pin driver, and is available in such advanced pin drivers as the AD345 pin driver from Analog Devices, Inc., Norwood, MS, U.S.A. In the AD345 pin driver, for example, the output stage is a pair of complementary emitter followers and a mode of operation is available in which a high impedance condition at the output is provided by reverse-biasing the bases of the output stage transistors to opposite supply voltages. This mode of operation is termed the "inhibit" mode. Operation in this mode, of course, disables and inhibits ordinary binary output switching operation.
Prior art pin drivers have been good, but leave room for improvement. For example, in the prior art, the delay time encountered in going into and out of the inhibit mode is not independent of the initial conditions. That is, it may take a substantially different amount of time to go between a logical high output (i.e., a digital "one") and a floating high impedance than it takes to go between a logic low output (i.e., a digital "zero") and a floating high impedance. Thus, the delay time may be quite variable. This variability of delay time interferes with obtaining precise measurements of DUT performance (e.g. time for the DUT to switch states or modes of operation).
Additionally, in prior art pin drivers, the output voltage can overshoot or preshoot the nominal logic levels when going into or out of inhibit mode. Both conditions are undesirable as they can lead to erroneous device or circuit operation. These characteristics are a direct result of reverse-biasing the output stage transistors to opposite supply voltages to create a high impedance output condition.